. The anatomy of the domestic animals . Veterinary anatomy. STRUCTURE OF BONES 21 STRUCTURE OF BONES' Bones consist chiefly of bone tissue, but considered as organs they present ne^eT ^'"''"^"P"'^' "^^"^^rane, termed the periosteum, the marrow, vessels, and ^'' ^""^i*""*''"! °^ ^°''' "^"^ ^"^ '*^^^'^ ^'^^^ by "^eans of longitudinal and transverse sections of specimens which have been macerated so as to remove most ol the organic matter. These show that the bone consists of an external shell of dense compact sub- sta
. The anatomy of the domestic animals . Veterinary anatomy. STRUCTURE OF BONES 21 STRUCTURE OF BONES' Bones consist chiefly of bone tissue, but considered as organs they present ne^eT ^'"''"^"P"'^' "^^"^^rane, termed the periosteum, the marrow, vessels, and ^'' ^""^i*""*''"! °^ ^°''' "^"^ ^"^ '*^^^'^ ^'^^^ by "^eans of longitudinal and transverse sections of specimens which have been macerated so as to remove most ol the organic matter. These show that the bone consists of an external shell of dense compact sub- stance, within which is the more loosely arranged spongy substance. In typical long bones the shaft IS hollowed to form the medullary cavity (Cavum medullare). The compact substance (Substantia compacta) differs greatly in thickness in various situations, in conformity with the stresses and strains to which the bone is subjected. In the long bones it is thickest in or near the middle part of the shaft and thins out toward the extremities. On the latter the layer is very thin, and is especially dense and smooth on joint surfaces. The spongy substance (Substantia spongiosa) consists of delicate bony plates and spicules which run in various directions and intercross. These are definitely arranged with regard to mechanical re- quirements, so that systems of pressure and tension plates can be recognized, in conformity with the lines of pressure and the pull of tendons and liga- ments respectively. The intervals between the plates are occupied by marrow, and are termed marrow spaces (Cellulae medullares). The spongy substance forms the bulk of short bones and of the extremities of long bones; in the latter it is not con- fined to the ends, but extends a variable distance along the shaft also. Some bones contain air-spaces within the compact substance instead of spongy bone and marrow, and hence are called pnetunatic bones (Ossa pneumatica). These cavities are termed sinuses, and are l
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